U.K. patent application No. 2,071,650, published Sept. 23, 1981, discloses .beta.-lactam antibiotics including (3S)-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amin o]-2-oxo-1-azetidinesulfonic acid, and 4-substituted derivatives thereof.